Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 212351

Area Forecast Discussion
National Weather Service Buffalo NY
751 PM EDT Wed Jun 21 2017

Weak high pressure will build into our region through tonight
while providing mainly dry weather and comfortable temperatures.
Broad low pressure will then slide eastward across central and
eastern Canada Thursday and Friday...while bringing a return to
increasingly unsettled conditions to close out the work week.
In the wake of this system...somewhat unsettled weather will
persist through the weekend as broad troughing lingers across
the Great Lakes.


An upper level trough axis has shifted to the east across New
England. Through early evening, there were a few showers
lingering across the St. Lawrence Valley, with otherwise dry
weather across the area. Diurnally driven cumulus have largely
dissipated, with the clearing trend expected to continue
tonight. With the loss of heating and diurnal clouds... skies
should range from mainly clear across far western New York to
partly cloudy across the far eastern Finger Lakes and North
Country... where some limited moisture will linger through the
night. Otherwise the dry/cool airmass and lightening winds will
be supportive of good radiational cooling...with overnight lows
ranging from the upper 40s across interior portions of the
Southern Tier to the lower and mid 50s elsewhere. As dewpoint
depressions narrow late tonight...some patchy light fog may
develop within the Southern Tier valleys.

On Thursday the surface ridge will slide off to our east. Meanwhile...
broad low pressure centered over Manitoba will slowly drift to western
Ontario province and push its attendant warm front northeastward
into our region...with this feature likely reaching the lower Genesee
Valley/western Finger Lakes by late afternoon or early evening. While
the approaching warm front will spread increasing amounts of mid and
high cloud cover across our region from southwest to northeast...
convective potential for Thursday afternoon remains far less certain
given both our position on the far eastern fringes of the warm front...
and poor agreement amongst the models on the timing/placement of
any associated precip. Given the above two this point
feel it best to just spread some broadbrush low chance PoPs eastward
across the area through the afternoon and early evening hours...with
the bulk of the day likely to be dry at most locations.

Temperature-wise...the steady warm air advection pattern across our
region on Thursday will allow afternoon highs to climb back to more
typical late June levels...with readings currently forecast to range
from the low-mid 70s east of Lake Ontario to the lower 80s across
far western New York. The above should be noted that the
above temps could still be on the conservative side for those parts
of far WNY that do break into the warm sector Thursday
850 mb temps will be in the mid to upper teens by that time...and
would easily be supportive of mid 80s provided both cloud and
convective coverage turn out to be low enough.


Thursday night a broad warm advection regime will remain in place
across the Lower Great Lakes, with another warm frontal segment and
associated subtle mid level shortwave moving ENE across the area.
The associated weak DPVA and isentropic ascent will maintain a
chance of a few showers and thunderstorms overnight, although the
weak/broad nature of the forcing will make the timing and placement
of better rain chances difficult at best. In a general sense expect
rain chances to increase through late evening and overnight as the
warm frontal segment arrives, then move northeast across Lake
Ontario and the North Country overnight. Clouds and warm advection
will keep temperatures very warm, with lows in the lower 70s on the
lake plains of Western NY and upper 60s for the interior Southern
Tier and east of Lake Ontario.

On Friday the warm sector will become established across the area.
It will not be a clean warm sector however, with a good deal of
cloud cover and scattered showers still around through the first
half of Friday. The best chance for breaks in the clouds will be
found from the Genesee Valley to Central NY from mid morning through
early afternoon. Later Friday afternoon and evening the cold front
will approach, with the associated mid level trough bringing
increasing DPVA and mid level height falls. At least some
instability will develop with a few breaks in the clouds, and
combine with increasing forcing and moisture convergence to produce
more widespread showers and thunderstorms during the afternoon and
evening. Southwest flow off the lakes may try to limit convection
across the Niagara Frontier and near Watertown, with the greatest
coverage likely from the western Southern Tier into the Finger Lakes
and southern Tug Hill region. It will become humid with dewpoints
rising into the 65-70 degree range. Expect highs in the lower to mid
80s at lower elevations away from lake influences, with mid to upper
70s across higher terrain and northeast of the lakes.

The somewhat limited instability and poor mid level lapse rates
should limit the overall severe potential, although an isolated
storm or two with strong winds cannot be ruled out. PWAT reaches
about 1.75 inches just ahead of the front, supporting heavy
downpours with a few storms.

Friday night the cold front will sweep east across the area, with
showers and thunderstorms ending from northwest to southeast. There
is enough of a mid level dry push to allow for partial clearing
overnight. Moderate cold advection and the arrival of a drier
airmass will allow temperatures to fall into the lower 60s on the
lake plains and upper 50s across the higher terrain.

On Saturday a broad longwave trough will be in place across the
Great Lakes, while a mid level shortwave moves across Southern
Ontario in the afternoon and brushes our region with some weak DPVA
and ascent. The airmass is fairly dry, so at first glance Saturday
appears to be mainly dry. Upon closer inspection, cool air aloft
will support steep lapse rates with daytime heating. The synoptic
pattern will support the development of a mesoscale convergence band
from Niagara County eastward along the south shore of Lake Ontario
all the way to Oswego County, as the enhanced WSW flow off Lake Erie
meets the WNW flow over Lake Ontario. This band of convergence
combined with the steep lapse rates may support a few scattered
showers and isolated thunderstorms Saturday afternoon, similar to
what we say Tuesday afternoon. Expect highs in the low to mid 70s in
most locations.

Any scattered showers will end Saturday evening as the convergence
zone breaks down and the boundary layer cools. This will leave
partial clearing overnight. Expect cool and comfortable lows in the
mid 50s on the lake plains and around 50 across the interior
Southern Tier and Lewis County.


Long range models are in good agreement on a overall broad troughing
pattern over the Hudson Bay over the weekend into early next week.
200hPa NAEFS height anomalies suggest a 99th percentile for the time
of year upper-level trough moving across the lower Great Lakes.
While the 850hPa temperature anomalies aren`t extreme, they are
certainly below normal as this trough crosses the region Monday into
Tuesday. More impressive are the 500hPa temperature anomalies,
suggesting steep mid-level lapse rates under the cyclonic flow
aloft. Typically in this pattern for this time of year we will see
an impressive diurnal cumulus field inland from the lake breezes
with showers and low topped thunderstorms developing each mid-day
through the afternoon. These showers and thunderstorms will be
further focused by any shortwaves rotating around the mean trough.
If this environment can be coupled with a stout shortwave, would
not rule out some small-hail producing storms given the lower
freezing level heights and steep lapse rates aloft, especially if
storms focus along a lake breeze boundary. Otherwise, this will
result in fairly cool and less humid conditions across the forecast
area with daytime highs perhaps not reaching the 70 degree mark
Monday and/or Tuesday as the trough axis swings through the area.


Expect VFR conditions tonight with the exception of valley fog
which may lower vsby to 3SM or less at times at JHW late
tonight. Mainly VFR conditions should persist Thursday, however
locally lower conditions are possible in scattered thunderstorms
as a warm front lifts from SW-NE across the region during the

Thursday night...VFR/MVFR with a few showers and thunderstorms
becoming likely.
Friday...VFR/MVFR with more widespread showers and thunderstorms.
Saturday and Sunday...Mainly VFR with a chance of showers and
Monday...Mainly VFR with a chance of showers.


High pressure will build across the Lower Great Lakes through
tonight. There will be light to modest winds and minimal waves
through much of Thursday...before increasing again for the
Thursday night and Friday as a broad low pressure system over
Canada pushes its attendant warm/cold fronts across the Lower
Lakes region. At this juncture...conditions appear as if they
will remain a little below advisory criteria through
Friday...with a better chance of advisory- worthy conditions
then arriving on Saturday as a somewhat stronger westerly flow
of cooler air overspreads the region in the wake of the cold





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